The special agent in charge of the FBI Boston office hopes someone somewhere heard something that will point to a suspect in the Boston Marathon attack.

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RICK DESLAURIERS: Any individual who expressed a desire to target the marathon: suspicious interest in researching how to create explosive devices, the noise of explosions in remote areas prior to yesterday, which may have been used as tests.

INSKEEP: That's the FBI's Rick DesLauriers.

GREENE: And NPR's counterterrorism correspondent Dina Temple-Raston has joined us in the studio now. Dina, good morning.

DINA TEMPLE-RASTON, BYLINE: Good morning.

GREENE: So we had some facts yesterday, not many. We knew there were two bombs in Boston, no more than that. Are we getting more physical evidence?

TEMPLE-RASTON: Yes. Because they haven't had the usual claims of responsibility, the FBI is focusing on physical evidence. And right now, among other things, they're focusing on the remains of the explosive devices that were used earlier this week.

We know that at least one of the devices is what is known as a pressure-cooker bomb. Basically, you take explosives and you put them in, like, one of those pots in which you cook rice. The other device was more damaged in the blast, so authorities are still trying to assess whether it was the same kind of bomb, or a slightly different one. Pieces of black nylon were also recovered, and officials think they might be part of a backpack or duffel bag that was used to bring the bombs to the finish line.

GREENE: All things you could buy almost anywhere.

TEMPLE-RASTON: That's right, but something traceable, which is important.

And then yesterday, if that wasn't enough, the FBI confirmed that there had been a letter sent to senator on the Hill that tested positive for the poison ricin.

INSKEEP: Which is immediately going to cause people to remember the weeks and months after 9/11, when there were letters sent to Capitol Hill. Are these events connected in any possible way?

TEMPLE-RASTON: The FBI has no reason to think that the two events are connected. They expect definitive tests on the ricin, which was sent to Republican Senator Roger Wicker of Mississippi, to come back as early as this afternoon. Now, ricin can be fatal if ingested or inhaled. And, as you said, you know, this is reminiscent of the anthrax attacks that came in the days after 9/11. So it's rattled people a bit.

INSKEEP: OK. So let's ask about possible suspects, even though there are no names that the FBI seems to have in their sights. Any indication in the evidence that the FBI has that al-Qaida might have anything to do with these attacks?

TEMPLE-RASTON: They've been very careful not to rule anything in or out. And pressure-cooker bombs have traditionally been the bomb of choice in Afghan jihadi training camps, which clearly has given investigators some pause. Directions for that kind of bomb appeared in an al-Qaida online magazine a couple of years ago. The magazine was called Inspire.

Now, that doesn't necessarily mean that this is a foreign plot or it has an al-Qaida connection, because anyone on the Web could have found the directions. In fact, a white supremacist website had linked to Inspire magazine and these directions, and told their followers that they could look for the recipe themselves.

INSKEEP: Groups are sharing information, you're saying.

TEMPLE-RASTON: Exactly. So it's not just Boston.

INSKEEP: A little detail here, Dina Temple-Raston. Yesterday, we were asking if, in fact, the day before, if cell phones had been used to detonate these explosives. Do you know how they were set off at this point?

TEMPLE-RASTON: Well, we're getting some indications. It doesn't look like it was a cell phone. It was some sort of timer. Again, forensics are going through these bombs and pawing through all this to see what exactly it is. But it may have been a kitchen or an egg timer.

GREENE: Dina, is one of the hard - the difficult things here that, I mean, if you're looking for a possible involvement of a foreign terrorist group, versus someone domestically? I mean, those sound like two incredibly different kinds of investigations. I mean, it's almost like they have to keep two tracks going at once in these early stages.

TEMPLE-RASTON: Well, when you have something like a bomb, there are signatures within a bomb. So there's a certain way that al-Qaida makes bombs. Basically, if you're bomb-maker and you haven't blown your hands off, you always twist to the wires three times and put green over red, because it worked. So there are lots of indications. When they have this kind of physical evidence, this goes a long way towards explaining who might be responsible. And these two tracks are very important because of these signatures.

INSKEEP: And it's interesting that you mentioned that there were directions to make this kind of bomb on the Web. You begin to wonder if there's going to be an electronic trail of some kind of any possible suspects.

TEMPLE-RASTON: Certainly, there looking at that. But it's early days for that, and right now what they have, you know, that they can actually point to is this physical evidence, because nobody has taken responsibility.

INSKEEP: And they are appealing to the public for evidence, for anything that people may have heard.

TEMPLE-RASTON: It's interesting. An FBI agent told me yesterday that they think the way they're going to solve this is going to be with some photograph or video that people don't know is important, but the FBI looks at and sees some clue.